The role of ferroportin in retinal iron homeostasis

Abstract

Age-related macular degeneration (AMD) is the leading cause of blindness in Western Nations. The etiology of AMD is multifactorial and has been associated with auto-immunity, the complement cascade, and oxidative stress. Iron is a potent generator of free radicals that contribute to oxidative stress. We have previously demonstrated increased iron levels in AMD-maculas and macular degeneration in a patient with aceruloplasminemia, an iron overload disease. Mice lacking the ferroxidases Ceruloplasmin (Cp) and Hephaestin (Heph) accumulate iron within the retina and develop retinal degeneration with features of AMD. Ferroportin (Fpn), an iron transport protein, functions with Cp/Heph to transport iron out of cells. Furthermore, the presence of Fpn on the cell membrane is regulated by hepcidin, a protein produced by both the liver and the retina. To study the role of iron accumulation in retinal degeneration and the function of Fpn within the retina, we undertook several studies. The expression of Fpn and response to hepcidin was studied in vitro using a human retinal cell line, ARPE-19. We used Polycythaemia and Flatiron mice to study changes in retinal iron homeostasis in mice that have genomic mutations in ferroportin. Finally, we developed a transgenic mouse line, BEST1-cre, that has retinal pigment epithelium specific ocular cre expression. These mice were use to generate RPE specific knockout of Fpn, and then analyzed for retinal degeneration, and changes in iron homeostasis. The following results suggest Fpn plays an important role in retinal iron homeostasis. Fpn protein and message increased in serum withdrawn ARPE-19 as they became more differentiated. Hepcidin treatment of ARPE-19 decreased cell surface Fpn levels, and, 60min after application decreased iron export and increased intracellular iron. Polycythaemia (Pcm) mice had an age-dependant thinning of the outer nuclear layer of the retina indicating that Fpn is important for retinal health. Pcm retinas had increased transferrin receptor immunofluorescence, suggesting retinal iron deficiency. Similarly, comparison of retinal gene expression in 6 month old Flatiron retinas vs. age-matched control showed an increase in transferrin receptor message, indicating that Flatiron retinas were iron deficient. BEST1-cre mice generated herein had RPE-specific cre expression causing the deletion of floxed sequences, in several mouse lines, including the floxed Fpn line. However, this line was not informative regarding RPE Fpn function in adult mice. Overall, experiments performed herein indicate that RPE ferroportin is regulated by intracellular iron levels and by hepcidin. It is important for retinal iron homeostasis and long-term photoreceptor survival